Dinosaurs

Dinosaurs are an extinct superorder of reptiles that first appeared approximately
230 million years ago. A few lines of primitive dinosaurs diversified
rapidly after the Triassic; the reign of dinosaurs encompassed the ensuing
Jurassic and Cretaceous periods. At the end of the Cretaceous, 65 million
years ago, nearly all species of dinosaur became extinct (the Cretaceous-Tertiary
extinction event), except for the line that had already led to the first
birds. There is now sufficient evidence to demonstrate that the birds
are the descendants of theropod dinosaurs.

The formal name Dinosauria was first proposed by the English scientist
Richard Owen in 1842. The term is a combination of the Greek words deinos
("terrible" or "fearfully great" or "formidable")
and sauros ("lizard" or "reptile").

Dinosaurs varied greatly in size. The smallest known species were about
the size of a chicken, but most were much larger. The biggest dinosaurs
were the Sauropoda; the species Argentinosaurus currently holds the record
for the largest land animals ever to live, and were second in size among
all creatures only to certain species of whale. However, the extraordinarily
massive femur of a brachiosaur, titled Ultrasaurus, suggests an animal
that could have weighed as much as 130 tons, dwarfing the competition.

Many other types of reptiles lived at the same time as the dinosaurs.
Some of these are commonly, but incorrectly, thought of as dinosaurs:
these include plesiosaurs (which are not closely related to the dinosaurs),
and Pterosaurs, which developed separately from reptile ancestors in the
late Triassic.

Dinosaurs are archosaurs, like modern crocodylians. These are set apart
by having diapsid skulls with teeth that grow from sockets, rather than
as direct extensions of the jaw bones, as well as various other characteristics.
Within this group, the dinosaurs are set apart most noticeably by their
gait. Instead of legs that sprawl out to the side, as found in lizards
and crocodylians, they have legs held directly under their body.